New experimental information is presented about the effect of a nucleus on patrons which have undergone a hard scatter. We have observed clear dijet events from 400 GeV/c p-nucleus interactions and compared them to dijets from pp interactions. Our results show that the angular width of the individual jets is only slightly affected by the presence of the nucleus, but the acoplanarity of the jet pairs is sharply increased.

Measurements are reported of the difference AuL between proton-proton total cross sections for parallel and antiparallel spin states and of the parameter CLL for proton-proton elastic scattering near 90", for thirteen energies between 300 and 800 MeV. The AaL results agree well with previous ANL ZGS and SIN data, but disagree with recent results from TRIUMF. Attempts to understand the cause of the discrepancy have been unsuccessful, but possible sources are discussed. The AUL and CLL results have been used with other experimental data to extract quantities which depend only on spin-singlet, coupled spin-triplet, and spin-triplet partial waves. Structure is found in these quantities, which appears to be associated with the resonantlike 'D2 and 3F3 partial waves. Additional similar structure is also found, which may be due either to the 'Po partial wave or the ( 3~2 , 3~2 ) partial-wave pair.

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PHYSICAL REVIEW LETTERS 16APRIL1979nals was estimated from single-particle inclusive data in which one arm was used as the trigger, and the other (untriggered) arm recorded the relative frequency of Cherenkov signals. 4 The original configuration of the apparatus is described by Bintinger et al. (Ref. 2), and the rearranged configuration (for which one of the Cherenkov counters was relocated behind the magnet) is described in Ditzler et al. (Ref. 2).Several unexpected results have recently raised interest in hadron production on heavy nuclei. The multiplicity of hadron-nucleus collisions grows with nuclear size much less rapidly than a simple cascade model would predict. 1 On the other hand, the inclusive production of high-/> x particles rises as A a with a significantly greater than l. 2 " 4 Theoretical models which attempt to describe this behavior include multiple scattering, nucleon clusters, and decay of high-mass states. 5 We report here the results of an experiment on the A dependence of dihadron production. 6 Only charged particles were detected, and for the purposes of this paper no distinction is made between 7T, K, and/>. The quantum-number correlations observed in lead and beryllium have been reported separately. 7 This experiment was performed at the Fermi National Accelerator Laboratory in a 400-GeV/c proton beam, with a typical intensity of 4x 10 7 sec" 1 . Other results and a detailed description 5 Our kinematic correlation results are reported in D. A. Finley etal., Phys. Rev. Lett. 42, 1031 (1979) (this issue). 6 M. G. Albrowet al. 9 Phys. Lett. 65B, 295 (1976). 7 R. J. Fisk et al. t Phys" Rev. Lett. 40, 984 (1978). 8 The observation that heavier nuclei reduce quantumnumber correlations as well as kinematic correlations (see Finley et al. t Ref. 2) is consistent with an inelastic multiple-scattering process.of the apparatus have been published previously. 8 The apparatus consisted of two identical magnetic spectrometers placed at 100 mrad on opposite sides of the beam. In the proton-nucleon centerof-mass system, each spectrometer was centered at 6 = 110° and subtended about ± 10° in polar angle, and ± 17° in azimuth. The trigger required each hadron to have£ ±^ 1 GeV/c.Measurements of the A dependence were made with a target of nine 1.3-mm lead segments followed by three 6.1-mm beryllium segments, all 3.8 mm wide. Data were taken on both nuclei simultaneously with targets of equal width in order to eliminate uncertainties arising from beam normalization or changes in experimental conditions. The good spatial resolution of the spectrometer drift chambers allowed unambiguous identification of the target element, as shown in Fig. 1. The acceptance of the spectrometer was uniform over the length of the target. The data were corrected for beam attenuation in the tar-We have measured the nucleon-number (A) dependence of hadron-pair production in 400-GeV/c proton-nucleus collisions, using Pb and Be targets. Charged-hadron pairs were observed near rapidity y Ca mr~ -0.4 with Acp s 180°. The A-dependenc...

could not be completely ruled out in the GMR experiment, are negligible here because we use low-energy positrons and because the o-Ps is essentially free of the surface. All of the results, including the powder data, are consistent with a vacuum decay rate of 7.09±0.02 Lisec" 1 , (2.1 ±0.3)% below the theoretical value.The advances discussed in GMR and this work lead to many possibilities for future research. In particular, the high signal-to-noise ratio and good statistics of the present work should make possible a 200-ppm measurement of A when the systematic effects are fully understood. Moreover, our method of detecting the time of the o-Ps formation may allow consideration of a pulsed excitation to the excited states. The Ps-atom and Ps-surface scattering can also be investigated. To help define the limits on the future use of powders in precision measurements, an attempt to observe the Ps ground-state fine-structure splitting in our uncompressed powders is now underway in collaboration with W. Frieze